Large grain polysilicon formation becomes required in an ever-increasing number of semiconductor devices.
Large grain polysilicon can be obtained with standard high thermal budget techniques like furnace annealing, e.g. rapid thermal annealing (RTA) or rapid thermal processing (RTP).
However, a general problem is obviously that in most cases the thermal budget is restricted to lower than 700° C., making it extremely hard to form large grain polysilicon from an amorphous silicon layer with the standard annealing techniques.
In an attempt to overcome the above problem, laser annealing, known to be a low thermal budget process, was proposed as being the most promising solution for low thermal budget polysilicon formation.
However, conventional laser annealing techniques for recrystallizing polysilicon do not allow formation of grain sizes above a few hundred nanometers, and since the carrier mobility is in proportion to amongst others the average grain size, the resulting laser annealed polysilicon shows average or low carrier mobility.
A special technique for forming low thermal budget large grains, as described in U.S. Pat. No. 7,029,996, uses laser annealing with a sequential lateral solidification process. This approach results obviously only in lateral grains, i.e grains extended in a direction parallel to the substrate surface area, and is generally limited to unpatterned polysilicon surfaces.
In addition, person skilled in the art will acknowledge that the equipment required for sequentially performing lateral displacements of the substrate (or the laser) appropriate for inducing lateral solidification is expensive and complex.
Considering the above, it is an object of the present invention to provide a method for forming large grain polysilicon with reduced equipment cost and complexity.
It is also an object of the present invention to provide a method for forming large grain polysilicon with increased throughput compared to state-of-the-art techniques.
It is also an object of the present invention to provide a method for forming large grain polysilicon with sufficient grain size while being exposed to limited thermal budget.
It is another object of the present invention to provide a method for forming large grain polysilicon allowing the formation of large grains in patterned polysilicon layers.
It is further a particular object of the present invention to provide a method for forming large grain polysilicon allowing formation of large vertical grains, i.e grains extended in a direction perpendicular to the substrate surface area.